Apparatus and method for duobinary transmission

ABSTRACT

An apparatus and method are provided for transmitting an optical duobinary signal using a low bandwidth modulator having a bandwidth of less than about 60% of the transmission bit rate of the transmitter. The modulator is adapted to provide low pass filtering for low pass filtered duobinary transmission in an optical fiber transmission system having residual dispersion.

REFERENCE TO RELATED APPLICATION

[0001] This application relates to a co-pending application entitled“Duobinary Transmission System And Method” filed even date herewith andis hereby incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to duobinary transmissionand, more particularly, to low pass filtered optical duobinarytransmission.

BACKGROUND OF THE INVENTION

[0003] Optical duobinary transmission is a well-known modulation formatin fiber optic communications. The duobinary transmission format ispotentially a cost effective commercial fiber optic data transportsolution, particularly for metropolitan applications. Characteristicsthat help make duobinary transmission potentially cost effective includea high tolerance to accumulated and/or residual dispersion, flexiblerequirements with respect to the placement of dispersion compensationunits within a transmission system, a high tolerance to nonlinearpenalty, and relatively low bandwidth requirements for the opticaltransmitter.

[0004] The optical duobinary transmission format transmits binary datausing three states, often denoted as plus-one (+1), zero (0), andminus-one (−1). The plus-one and minus-one states are differentiated bya 180 degree optical phase shift. An optical duobinary data stream istypically created by driving a single Mach-Zehnder modulator (MZM) witha three level electrical drive. The modulator is biased at the nullpoint in its transfer function and driven with an electrical signal thathas three levels, where the upper and lower rails of the drive signalare separated by two times the required switching voltage of themodulator. This creates a three state optical output from the modulatorwhere the upper and lower rails of the electrical drive signal produce aplus-one and minus-one state, respectively, and the middle state of theelectrical drive signal creates a zero state from the modulator.

[0005] One technique for realizing an optical duobinary data stream isto create the required three-level electrical drive signal, used todrive the optical modulator, by filtering a differentially encodedbinary NRZ data stream with a low pass electrical filter. This techniqueis referred to herein as the electrical low pass filtered (LPF)duobinary approach. An example of an optical transmission system usingconventional electrical LPF duobinary is shown in FIG. 1.

[0006] Electrical LPF duobinary transmission uses an opticalMach-Zehnder modulator (MZM) biased at a null point in its transferfunction and driven at about two times its required switching voltage(2*V). In such a configuration the required response bandwidth of thedriver/filter/modulator combination to an input impulse is much lowerthan that needed for an NRZ transmitter. However, for optimalperformance in, for example, a 10 Gb/s electrical LPF duobinarytransmitter, a 3 GHz bandwidth first-order Gaussian low pass electricalfilter is used as the ‘bandwidth bottleneck’ or bandwidth limiter. Thebandwidth of the modulator and electrical driver in such a configurationare made to be significantly larger than the Gaussian electrical filterpass band in order to let the carefully designed filter create anappropriate spectral content for the data stream. Therefore, the 3 GHzelectrical filter response dominates the transmitter response and isindicative of the preferred aggregate driver/filter/modulator responseof the transmitter. Significant deviations from this ideal response, inbandwidth and/or response ripple, tend to seriously degrade the qualityof the signal at the output of the transmitter. Therefore, great care istaken in achieving the proper transmitter response for use in acommercial system.

[0007] It is important to note that, within typical electrical LPFduobinary transmitter circuits, impedance matching must be maintainedbetween various components within the circuit including the amplifier,electrical filter and modulator in order to minimize signal reflections.Reflections between the electrical filter and modulator, for example,can seriously degrade the back-to-back performance of a filteredduobinary transmitter. (Back-to-back performance of the duobinarytransmitters as used herein, refers to the quality of the data stream atthe output of the transmitter without transmission across a transmissionlink.) To avoid signal reflections great care must be taken to minimizeimpedance mismatches within the transmitter. Such stringent transmitterspecifications tend to substantially increase transmitter componentcosts. Accordingly, relaxing these constraints on system architecturewould significantly improve transmitter cost and yield.

[0008] It has been demonstrated that an approximate filtering functionfor realizing a duobinary data stream can be created within the responseof a Mach-Zehnder electro-optic modulator (see Enning, “Signal ShapingFor Optical Wideband Transmission Systems Using Inherent LowpassBehavior of Counterpropagating Optical and Electrical Signals in aLiNbO₃ Mach-Zehnder Modulator”, J. Opt. Commun. 22 (2001) 746 pp. 1-5,2001). The Enning device employs an idealized magnitude sinc functionresponse for the modulator. However, such a device response is notrealistic for practical devices or for higher bit rate (e.g. ˜10 Gb/s)applications. The use of a modulator with a sinc magnitude response hasa monotonic decrease in the quality of the data pattern with an increasein residual dispersion. Therefore, the sinc response modulator creates adata pattern that is not as robust against accumulated dispersion withina transmission link.

[0009] It has also been recognized that residual dispersion fromtransmission can improve the quality of an electrical LPF duobinary datastream. Specifically, it is understood that the optical spectralcomponents that make up an electrical LPF duobinary data stream are notoptimally aligned when initially transmitted; however, after propagationin standard single mode fiber (SSMF) dispersion can realign some of thespectral components within the bit stream such that the eye diagramimproves, resulting in an improved optical signal to noise ratiorequirement for a given bit error ratio. However, it has not previouslybeen understood that the amount of improvement in the electrical LPFduobinary data stream produced by residual dispersion variessignificantly with the initial quality of the bit pattern. That is, ithas not been previously recognized that a lower quality duobinary datastream generated using a transmitter having a low bandwidth modulatorcan be dramatically improved with an appropriate amount of residualdispersion, and that a relatively high quality duobinary data streamshows significantly less improvement.

[0010] Accordingly, a need exists for an optical duobinary transmitterand method for optical duobinary transmission for higher bit rateapplications, which are practical, relatively less technically complexand are cost effective.

SUMMARY OF THE INVENTION

[0011] One aspect of the invention provides an optical duobinarytransmitter comprising a precoder, an amplifier coupled to the precoder,and a low bandwidth Mach-Zehnder modulator coupled to the amplifierhaving a bandwidth of less than about 60% of the transmission bit rateof the transmitter. The modulator is adapted to provide low passfiltering for low pass filtered duobinary transmission in an opticalfiber transmission system having residual dispersion.

[0012] Another aspect of the invention provides a method for opticalduobinary transmission comprising transmitting an optical duobinarysignal using a low bandwidth modulator means having a bandwidth of lessthan about 60% of the transmission bit rate of the transmitter, themodulator means being adapted to provide low pass filtering for low passfiltered duobinary transmission in an optical fiber transmission systemhaving residual dispersion.

BRIEF DESCRIPTION OF THE DRAWING

[0013]FIG. 1 is a schematic diagram of a prior art electrical LPFduobinary transmitter and transmission link;

[0014]FIG. 2 is a schematic diagram of one embodiment of a duobinarytransmitter according to the invention;

[0015]FIG. 3 is a plot showing the simulated magnitude response of aMach-Zehnder modulator (solid line) and a low pass 3 GHz Gaussianelectrical filter (dashed line);

[0016]FIG. 4 is a plot showing the phase response of a Mach-Zehndermodulator according to one embodiment of the invention and the phaseresponse from an ideal low pass 3 GHz Gaussian electrical filter;

[0017]FIGS. 5a-b show eye diagrams from simulations using a standardelectrical LPF duobinary transmitter and a duobinary transmitteraccording to an embodiment of the present invention, respectively;

[0018]FIG. 6 is a plot showing the measured magnitude response oflow-bandwidth modulators compared to an ideal low pass 3 GHz Gaussianelectrical filter;

[0019]FIG. 7 is a plot showing the measured phase response oflow-bandwidth modulators; and

[0020]FIG. 8 is a plot showing required OSNR vs. transmission distancefor duobinary optical transmitters in accordance with the presentinvention and an electrical LPF duobinary transmitter.

DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention provides an apparatus and method fortransmitting optical duobinary signals, which allow for relaxed responserequirements and fabrication tolerances for an optical duobinarytransmitter for duobinary transmission across transmission links havingresidual dispersion. Residual dispersion as used herein refers todispersion accumulated in a transmission link up to a point just priorto a receiver.

[0022] In one preferred embodiment of the invention, shown in FIG. 2, aduobinary transmitter 210 is provided which includes a differentialencoder 220, an RF amplifier 230, a Mach-Zehnder modulator 240, and alaser 250.

[0023] As shown in FIG. 2, the transmitter 210 may be coupled to one ormore transmission links and a receiver. ‘Transmission links’ as usedherein, refers to transmission apparatus or system components between atransmitter and a receiver that are necessary or appropriate for adesired application. Such components include but are not limited totransmission fiber, optical amplifiers, dispersion compensating modules,optical add-drop units and the like.

[0024] The differential encoder 220 of the transmitter 210 is used toconvert an electrical binary input signal to a differentially encodedsignal that is then used to drive an appropriately configuredMach-Zehnder modulator 240 to produce an optical duobinary signal.

[0025] The RF amplifier 230 is provided to amplify the duobinary signalprior to driving the modulator 240. It is understood that the RFamplifier 230 is needed only if the power output from the differentialencoder 220 is not sufficient to drive the modulator 240. The RFamplifier 230 can be removed entirely from the transmitter 210architecture if the differential encoder 220 output is sufficient todrive the modulator 240.

[0026] The modulator 240 is arranged to modulate the optical signalprovided by the laser 250 according to the driving signal suppliedthrough the differential encoder 220 and the RF amplifier 230. Themodulator is biased at a null point, and the driving signal is low-passfiltered by the modulator response function in such a way that themodulated optical signal is a duobinary signal.

[0027] Preferably, the duobinary transmitter response needed forduobinary transmission is realized using the modulator 240. As can beunderstood from FIG. 1, prior art electrical LPF duobinary transmittersinclude electrical low pass Gaussian filters having a 3-db bandwidth (inGHz) of approximately ¼ of the bit rate (in Gb/s) that creates therequired filtering within the duobinary transmitter. In contrast, theduobinary transmitter 210, in accordance with an embodiment of thepresent invention, uses the modulator 240 to limit the frequencyresponse of the transmitter 210. The modulator 240 is preferably a lowbandwidth modulator having a bandwidth (in GHz) of less than thetransmission bit rate (in Gb/s) of the transmitter, that is adapted toprovide an appropriate response such that Gaussian electrical filteringis not necessary within the transmitter 210 architecture. Morepreferably, the modulator 240 has a bandwidth of less than about 60% ofthe transmission bit rate of the transmitter 210. Specifically, themodulator 240 preferably indicates the lower limit of the allowablefrequency response of the transmitter 210. The use of such low bandwidthmodulators allows for the use of lower modulator drive voltages. Such anarchitecture provides a practical cost-effective solution for duobinarytransmission.

[0028] It should be understood, however, that the present invention isnot limited to transmitters without electrical filters, but is insteadapplicable to any transmitter in which the modulator is adapted toprovide filtering for low pass filtered duobinary transmission withoutthe need for additional electrical filtering which dominates theresponse of the transmitter.

[0029] As an example, we consider a 3 GHz bandwidth first-order Gaussianlow pass electrical filter response 310, shown in FIG. 3, as is used intypical electrical LPF duobinary transmitters for 10 Gb/s transmission.For LPF duobinary applications the preferred magnitude response 310 isas shown in FIG. 3, and the preferred phase response 410 is flat acrossthe pass band of the filter, as shown in FIG. 4.

[0030] A simulation which modeled 10 Gb/s optical duobinary transmissionwas performed using a transmitter having a low-bandwidth Mach-Zehndermodulator in accordance with one embodiment of the invention. It isunderstood, however, that transmitters according to the invention can begeneralized to optical duobinary transmitters for transmission atvarious bit rates (e.g. 40 Gb/s) by scaling the modulator bandwidthproportionally.

[0031] The response function 320 of the low-bandwidth modulator is shownin FIG. 3. The physical parameters of the modulator used in thesimulation are as follows: n_RF=3.7, n_opt=2.1385, device length=2.5 cm,RF_loss_factor=−0.7 dB/cm/sqrt(cm). The high frequency response of thelow-bandwidth modulator is predominantly limited by the velocitymismatch between the microwave and optical field propagation constantsin the modulator interaction region. However, the electrode loss plays alarger role in the frequency response of the device below 1 GHz. As canbe seen from FIG. 3, there is a deviation between the Gaussian 310 andthe modulator response 320 below 1 GHz. Furthermore, we note that thebandwidth of the modulator response shown in FIG. 3 (i.e. the frequencyat which the magnitude response of the modulator is reduced by 3 dB) is˜2 GHz.

[0032] The phase response 420 of the low-bandwidth modulator is shown inFIG. 4. We note that since group delay is the rate of change of thetotal phase shift with respect to angular frequency, the phase responseof the modulator can be plotted with the subtraction (or addition) of afactor that is linearly dependent on frequency. This, in essence, isequivalent to creating a time delay (or advancement) in the measuredoutput of the modulator. Accordingly, the phase response of thelow-bandwidth modulator with the addition of a factor that increaseslinearly with frequency 430, is also shown in FIG. 4.

[0033] As can be seen from FIG. 4, the phase response 430 of thelow-bandwidth modulator has relatively small (±5 degrees) deviation froman ideal linear response 410. Thus, the major impact that the modulatorphase response has in the frequency range of interest (e.g. <5 GHz) isto produce a time delay in the output data stream from the modulator,and that the output data stream from the modulator is otherwise leftrelatively intact. Thus, the response of the modulator substantiallyreplicates the ideal filter function needed for duobinary transmission,except for the magnitude response deviation below 1 GHz caused byelectrode loss, which could be corrected by some other effects such asfrequency-dependent impedance matching. Even with this deviation (andother deviations), residual dispersion may mitigate the penalty.

[0034]FIGS. 5a-b show eye diagrams from simulations of the lineartransmission of a 10 Gb/s duobinary data stream created with anelectrical LPF duobinary transmitter (with a 3 GHz filter) (FIG. 5a),and a duobinary transmitter (with a low bandwidth MZ modulator) inaccordance with one embodiment of the present invention (FIG. 5b), afterpropagation through 100 km of standard single mode fiber. The RF driverbandwidth of the duobinary transmitter according to the invention wasset such that it is much larger than that of the modulator so that theduobinary transmitter response is dominated by the Mach-Zehndermodulator response. As can be seen from FIGS. 5a-b, the resulting eyediagrams after 100 km of transmission on standard single mode fiber aresimilar for both the transmitter using electrical filtering and thetransmitter according to an embodiment of the present invention.

[0035]FIG. 6 shows the measured response 610, 620 from representativelow-bandwidth modulators that can be used in accordance with embodimentsof the invention for 10 Gb/s optical duobinary transmission. As can beseen from FIG. 6, the response curves 610, 620 deviate appreciably fromthe response curves 310, 320 shown in FIG. 3 (e.g. magnitude responseripples) and from the ideal response 630 for the 3 GHz low pass Gaussianfilters traditionally used for 10 Gb/s duobinary transmission withelectrical LPF duobinary transmitters having high bandwidth modulators.

[0036] The phase response 710, 720 from the low-bandwidth modulatorsdiscussed with regard to FIG. 6 are shown in FIG. 7. As can be seen fromFIG. 7, the curves 710, 720 have some irregular phase ripples.

[0037]FIG. 8 illustrates the optical signal to noise ratio (OSNR)required to achieve a bit error ratio (BER) of 1E-3 for three 10 Gb/stransmitters, for transmission distances up to 200 km of SSMF. Plot 810is for a prior art 10 Gb/s electrical LPF duobinary transmitter having a10 Gb/s MZ modulator. Plots 820 and 830 are for 10 Gb/s duobinarytransmitters having low-bandwidth modulators in accordance withembodiments of the invention. As can be understood from FIG. 8, theplots show the required OSNR for varying transmission distances and,accordingly, varying amounts of residual dispersion. The required OSNRfor back-to-back measurements (i.e. ˜0 km of transmission) shows asignificant difference of as much as ˜1.5 dB. However, at ˜100 km ofSSMF (e.g. ˜1700 ps/nm of residual dispersion) the difference inrequired OSNR between the electrical LPF duobinary transmitter and theduobinary transmitters in accordance with the present invention issignificantly reduced to only about 0.5 dB. Furthermore, the duobinarytransmitters in accordance with the present invention perform well ascompared to prior art electrical LPF duobinary transmitters where theresidual dispersion in a transmission link is between about 600 ps/nmand about 3000 ps/nm.

[0038] It is understood that the OSNR penalty from the use of aduobinary transmitter in accordance with the present invention is due,at least in part, to the response deviations of the transmitter from theideal duobinary transmitter response. Simulations using a duobinarytransmitter in accordance with the present invention indicate that aresponse deviation of up to a ±1 dB in the low frequency regime of theduobinary transmitter can be tolerated with a relatively small amount ofOSNR penalty within a range of residual dispersion from about 600 ps/nmto about 3000 ps/nm. Although a significant back-to-back OSNR penaltycan be seen from a low frequency ±1 dB response deviation, after anappropriate amount of residual dispersion, the OSNR penalty is seen tobe minimal.

[0039] It can be appreciated by those skilled in the art that theimprovement in a duobinary data signal with residual dispersion from atransmitter according to the invention can reduce the impact from avariety of issues that may produce a less than ideal transmitterresponse. For example, with regard to impedance matching between variouscomponents within a duobinary transmitter, the impedance matchingrequirements between various components can be relaxed since theresulting signal distortion can be mitigated with the presence ofresidual dispersion within the transmission link. This is of specificinterest in matching modulators with electrical drivers in duobinarytransmitters since impedance variations can exist between modulatorsmanufactured by different vendors.

[0040] Further, a significant amount of ripple in the modulatorresponse, as was seen in the low-bandwidth modulators discussed above,can be tolerated as long as there is an appropriate amount of residualdispersion in the transmission link.

[0041] This is significant in that the aggregate response of a duobinarytransmitter according to the present invention need not be nearly asideal as in prior art devices.

[0042] Thus, transmitters according to the present invention can, forexample, transmit a 10 Gb/s duobinary signal that is acceptable forcommercial transmission systems, using low-bandwidth modulators. Suchmodulators may be manufactured under relaxed fabrication and packagingconstraints. This significantly reduces the cost of modulators used inthe duobinary transmitters while maintaining performance that isacceptable for commercial 10 Gb/s transmission applications.Furthermore, the performance of the modulators used in accordance withthe present invention may be further relaxed from those of presentlyavailable low-bandwidth devices to further reduce costs and improveyield.

[0043] Additionally, an optical duobinary transmitter according to theinvention may have relaxed transmitter response criteria whilemaintaining good performance. Use of relaxed transmitter specificationsfor duobinary transmission at any bit rate can allow significantreduction in modulator costs and lower modulator drive voltagerequirements. Therefore, a more efficient and cost effective approach toimplementing duobinary transmission can be realized.

[0044] The present invention can be implemented using discretecomponents or using integrated modules (e.g. laser/modulator modules,duobinary driver/modulator modules, laser/duobinary driver/modulatormodules, and the like).

[0045] The above-described embodiments of the invention are intended tobe illustrative only. Numerous alternative embodiments may be devised bythose skilled in the art without departing from the scope of thefollowing claims.

we claim:
 1. An optical duobinary transmitter apparatus comprising: aprecoder; an amplifier coupled to the precoder; and a low bandwidthmodulator coupled to the amplifier having a bandwidth of less than about60% of the transmission bit rate of the transmitter, wherein themodulator is adapted to provide low pass filtering for low pass filteredduobinary transmission in an optical fiber transmission system havingresidual dispersion.
 2. The apparatus of claim 1 wherein the lowbandwidth modulator is a Mach-Zehnder modulator.
 3. An optical duobinarytransmitter apparatus comprising: a low bandwidth modulator having abandwidth of less than about 60% of the transmission bit rate of thetransmitter, wherein the modulator is adapted to provide low passfiltering for low pass filtered duobinary transmission in an opticalfiber transmission system having a predetermined amount of residualdispersion, such that the residual dispersion substantially compensatesfor signal distortions from the low bandwidth modulator.
 4. Theapparatus of claim 3 wherein the low bandwidth modulator is aMach-Zehnder modulator.
 5. The apparatus of claim 3 wherein signaldistortions from the low bandwidth modulator include substantialresponse ripple.
 6. A method for optical duobinary transmissioncomprising: providing a precoder means; providing an amplifier meanscoupled to the precoder; providing a low bandwidth modulator meanscoupled to the amplifier means having a bandwidth of less than about 60%of the transmission bit rate of the transmitter; and adapting themodulator means to provide low pass filtering for low pass filteredduobinary transmission in an optical fiber transmission system havingresidual dispersion.
 7. The method of claim 6 wherein the low bandwidthmodulator is a Mach-Zehnder modulator.
 8. A method for optical duobinarytransmission comprising: transmitting an optical duobinary signal usinga low bandwidth modulator means having a bandwidth of less than about60% of the transmission bit rate of the transmitter; wherein themodulator means is adapted to provide low pass filtering for low passfiltered duobinary transmission in an optical fiber transmission systemhaving residual dispersion.
 9. The method of claim 8 wherein the lowbandwidth modulator is a Mach-Zehnder modulator.
 10. An opticalduobinary transmitter apparatus comprising: a precoder means; anamplifier means coupled to the precoder; and a low bandwidth modulatormeans coupled to the amplifier means having a bandwidth of less thanabout 60% of the transmission bit rate of the transmitter, wherein themodulator means is adapted to provide low pass filtering for low passfiltered duobinary transmission in an optical fiber transmission systemhaving residual dispersion.
 11. An optical duobinary transmitterapparatus comprising: a low bandwidth modulator means having a bandwidthof less than about 60% of the transmission bit rate of the transmitter,wherein the modulator means is adapted to provide low pass filtering forlow pass filtered duobinary transmission in an optical fibertransmission system having a predetermined amount of residualdispersion, such that the residual dispersion substantially compensatesfor signal distortions from the low bandwidth modulator.